The present invention relates to a process and apparatus for the catalytic treatment of materials containing solid contaminants.
The run length of plants operating catalytic processes having feedstocks contaminated with solid particles is very often limited by the maximum allowable pressure drop across the catalytic reactor(s) and not by the decline in activity of the catalyst. During operation of the pressure drop across the reactor(s) gradually increases owing to solid particles blocking the interstices between the catalyst particles, through which the material would otherwise flow, until it finally reaches a level above which economic operation is no longer possible. Although the catalyst then gradually has not lost its activity, yet the plant must be shut down in order to either regenerate the solids-contaminated catalyst in situ -- if at all possible -- or to remove part or all of it and replace it with fresh catalyst, both of which is expensive. If the feedstock is heavily contaminated with solid particles the increase in pressure drop across the reactor(s) occurs rapidly. Accordingly, the lengthy shut-down periods for catalyst replacement/regeneration are more frequent than would be required by the decline in catalyst activity per se. This is, for obvious reasons, both inconvenient and uneconomic. One method by which catalyst replacement or regeneration may be deferred is to pass the contaminated feedstock through one or more filter units before passing it to the catalyst reactor(s). This, however, involves additional, normally high-pressure equipment which adds significantly to the capital cost of the plant. It is therefore an object of the present invention to prolong the period between catalyst replacements/regenerations of a plant operating a catalyst process having a feedstock contaminated with solid particles in a simple and economic manner.
Particular feedstocks envisaged by the present process are hydrocarbon materials, and a particular catalytic process in which a rapid increase in pressure drop across the catalytic reactor(s) may occur is the hydrodesulphurization of residual oils since such oils usually contain relatively large amounts of solid particles, and may form additional quantities of such particles upon heating to treatment temperature. In addition to solid particles, however, they also contain to a greater or lesser extent, metal contaminants which have a deleterious effect on the activity of the catalyst, which even may quickly become virtually unregenerable. Accordingly, in cases where the residual oil contains large amounts of metal contaminants as well as of solid particles it is desirable to remove both these unwanted constituents to a substantial degree before passing the residual oil over the catalyst bed(s). To this end it has previously been proposed to pass the residual oil through a catalyst guard reactor, either provided with a fixed catalyst bed, or into which fresh catalyst is continuously or intermittently being introduced and from which spent catalyst is continuously or intermittently being withdrawn, prior to passing it to the main hydrodesulphurization reactor(s). In this way a continuous stream of substantially demetallized, substantially particle-free residual oil is passed to the main hydrodesulphurization reactor(s). A fixed bed guard reactor is normally provided with a twin, which can be connected to the main reactor whenever the pressure drop across the first guard reactor builds up to an unacceptable level; with additional and removal of catalyst, this pressure drop builds up to an equilibrium level and does not reach a level at which operation becomes uneconomic. However, both these embodiments require expensive equipment such as high-pressure vessels and switching valves and/or valves for (particulate) solids and require careful operational control; hence, especially in cases where the residual oil contains only a small amount of metal contaminants and a substantial amount of solid particles a more simple and economical method of removing the solid particles is desirable. It is therefore another object of this invention to provide a process for the hydrodesulphurization of residual oils containing substantial amounts of solid particles in which the catalyst need not be replaced or regenerated for a long period of time.